The invention relates to an image sensor device for receiving a radiation image and converting same into an electric signal, comprising a semiconductor body having a surface layer of mainly one conductivity type in which a row of photo-sensitive elements is present which are each capable of absorbing incident radiation and converting it into charge carriers which can be stored in the photosensitive elements during a certain time interval, hereinafter termed frame time, and means for reading out the charge carriers stored in the photosensitive elements, said means comprising a charge transfer register having a series of electrodes which are provided on the surface of the layer, are separated from the layer by a barrier junction and form with the underlying semiconductor material a series of capacitances in which the information in the form of charge packets obtained in the photosensitive elements by generation of the charge carriers can be shifted to a read-out member, the device further including means for transferring the charge carriers in the photosensitive elements to the charge transfer register after each frame time.
The invention moreover relates to a photosensitive element suitable for use in an image sensor device as described above.
Image sensor devices based on the charge transfer principle are generally known. Such devices can be read electrically, instead of by means of an electron beam, by step-wise moving through the register the charge packets each containing information about the quantity of radiation which has locally been absorbed and sequentially reading at the output of the register. The charge transfer register may be formed, for example, by a so-called bucket brigade register, or by a charge coupled device.
Devices of the kind described above in which the photosensitive elements and the charge transfer register are separated from each other are sometimes referred to as image sensor devices of the interline type. A matrix structure of this type comprises a number of lines of photosensitive elements. Adjacent each line is an associated charge transfer register. These devices are distinguished from another type of image sensor device in which the functions of charge transfer and photosensitive element are not separated but in which the charge transfer register also provides the photosensitive elements. Separation of these functions, however, has several advantages. In particular, in a device of the interline type it is possible to obtain accurately defined integration times by simultaneously transferring, after each frame time, the charge packets stored in the photosensitive elements to the charge transfer register which can be screened against radiation. In addition, the parameters of the photosensitive elements and of the charge transfer register can be chosen more independently of each other, which may be very advantageous for improved performance of the device.
In spite of this, the possible choices of parameters of the photoresistive elements in this type of device are often still too restricted than would sometimes be desirable for an optimum operation of the device. For example, it is known to use photosensitive elements having an insulated gate electrode on the surface of the body which is insulated from the semiconductor material by an intermediate insulating layer of, for example, silicon oxide. By means of this insulated gate electrode a depletion region in which, or in the proximity of which, charge carriers can be generated and stored by absorption of radiation can be induced in the underlying semiconductor region. The sensitivity of such a device, however, can be detrimentally influenced by the insulated gate electrode when exposure is through the electrode surface. When a metal layer is used as an insulated gate electrode, the layer should in general be very thin since otherwise it becomes opaque to radiation. The provision of such thin metal layers often requires an extra manufacturing step during the production process.
The use of a semiconductor material, for example polycrystalline silicon, instead of metal has the advantage that the sensitivity of the device can be improved over a broad spectral range. Since, however, the absorption coefficient of silicon for shorter wavelength radiation is comparatively high, only a slight improvement for blue light can be obtained.
Absorption (and/or reflection) by the insulated electrodes can be prevented by receiving the radiation on the rear side of the semiconductor body. However, this usually requires an extra step during the production process to (at least locally) remove the semiconductor material on the rear side, for example by etching, to such an extent that incident radiation can penetrate into or at least near the depletion region induced by means of the insulated electrodes.
Instead of photosensitive elements having insulated electrodes, photosensitive diodes may also be used in the form of zones of the second conductivity type opposite to that of the semiconductor. These zones form photosensitive p-n junctions with the semiconductor layer, for example, as is usual in a silicon vidicon. The diodes can be charged electrically by applying a voltage in the reverse direction across the p-n junctions and then be discharged by absorption of incident radiation, charge carriers being generated which can provide information on the (local) intensity of the radiation.
However, photodiodes have the drawback that their charge storage capacity is often comparatively small, which means that less charge can be stored in the diodes than would be desirable. The principal reason for this is that the electric field strength across the insulating layer, for example silicon oxide, which in most of the cases is decisive of the breakdown in the charge transfer register, can in general be larger than the field strength in the semiconductor material itself. Avalanche multiplication of the semiconductor material generally occurs with comparatively low fields, as a result of which the favorable effect of the comparatively large dielectric constant on the diode capacitance is nullified entirely.
Increase of the charge storage capacity of the photo diodes by enlarging the area of the p-n junctions often results in an undesired decrease of the resolving power of the image sensor device and/or in an undesired enlargement of the semi-conductor body.